Mr Blair owes it to the country to spell out his conditions for war

Not for the first time, Clare Short has spoken with admirable clarity. She is quite right to strip the Prime Minister's recent statements of their evasions and say that "the logic of the position" is that, if the United States decides to take unilateral military action against Iraq, the British government should not support it.

Tony Blair has appeared to lose his grip of events, because he has tried to keep his options open for too long. He desperately wants to avoid saying what Ms Short said yesterday, because he hopes it will never come to that. But that is certainly the implication of what he told the House of Commons last week.

He was asked by Charles Kennedy of the Liberal Democrats the same question that Ms Short answered, and said he was "not going to speculate on the circumstances that might arise", but that "both Britain and the US have made it clear that we seek to resolve the matter through the UN."

That was an important choice of words, because in answer to the same hypothetical question during the recalled session of Parliament last September, he said that "the American relationship and our ability to partner America" was "an article of faith with me", which set a quite different tone.

The problem is that Mr Blair has never set out the conditions under which British forces should go to war. We know a lot about what a terrible man Saddam Hussein is, and how he has defied UN resolutions in the past. But we do not know what has to happen to make the use of military force necessary in Mr Blair's eyes. All we know is that sticking with the US was an article of faith with him, and that he does not regard a single country's veto on the UN Security Council as a bar to action.

We also know that public opinion in this country is far from persuaded of the need for military action, and that the Labour Party is overwhelmingly hostile. The Prime Minister cannot carry on much longer saying that nothing has been decided, when the US has a quarter of a million troops on the move and British forces are themselves being readied.

The mixed British signals were still being sent at the weekend with the departure from Portsmouth of the Ark Royal – only for it to emerge towards the end of the news bulletins that the carrier's first port of call would be in Scotland.

It may be, as the Labour Party chairman, John Reid, said yesterday, quoting Mr Blair who was himself quoting Flavius Vegetius Renatus, that "if you want peace you had better prepare for war." But it is difficult to believe that the US-British mobilisation is purely a precautionary measure to let Saddam know the UN is serious.

Mr Blair tries to make this case. The choice of war is Saddam's, he says. But it is hard to believe him while the American momentum for war goes unchecked by any public words of caution from America's First Friend. So long as Mr Blair refuses to allow any public difference of opinion between him and the US President, he cannot persuade his party or his people that he is sincere in wanting to use force if and only if a coalition of nations decides that it is the only way to avert something worse.

When he addresses his MPs on Wednesday, Mr Blair ought to recognise that the seriousness of the moment means that he can no longer dismiss "what if" questions as speculation. Keeping options open is the elementary tactic of any successful politician. But it cannot be maintained indefinitely against the drumbeats of war.